cells - MCAT

credit to khan academy

Describe the function of cholesterol in animal cell membranes.

Acts as buffer for fluidity

At high temperatures, membrane tends to become more fluid; cholesterol helps maintain rigidity

At low temperatures, membrane tends to become more rigid; cholesterol helps maintain fluidity

Why is the assembly of a cell membrane from free phospholipids spontaneous?

Free phospholipids are surrounded by water in an orderly fashion

When they assemble into a bilayer, it disturbs that orderly arrangement, increasing entropy which increases thermodynamic favorability of the process

What is the function of saturated and unsaturated fatty acid tails in the phospholipid bilayer?

Saturated fatty acids allow for tight packing, increasing membrane rigidity/integrity and conferring heat resistance

Unsaturated fatty acids prevent tight packing, increasing membrane fluidity and conferring freeze resistance

Define and compare: Simple diffusion, Facilitated diffusion, Primary active transport, Secondary active transport

Simple diffusion = movement of small nonpolar molecules along concentration gradients

Facilitated diffusion = movement of polar molecules and ions along concentration gradients using channels

Primary active transport = movement of ions against concentration gradient using ATPase pump

Secondary active transport = movement of molecule with positive ion against concentration gradient (created by primary active transport)

3 types of endocytosis

Phagocytosis for undissolved molecules

Pinocytosis for dissolved solutes

Receptor-mediated endocytosis for engulfing ligand + receptor

Phagocytosis

endocytosis of undissolved / insoluble molecules

1) contact made and surface receptors bind

2) pseudopods reach around and enclose food in membrane → phagosome

3) phagosome + lysosome = phagolysosome → acid digestion

3 main cell-cell connections

Desmosomes = sticky webs → impermeable connections with space between cells

Tight junctions = cell membranes connected into impermeable barrier

Gap junctions = tubes that hold cells together

Plasmodesmata

Gap junctions for plants

What is a G-protein coupled receptor, and how does it work?

Receptor protein with 7 transmembrane alpha helices

Start with receptor bound to G-protein’s alpha subunit bound to GDP

Ligand binds → conformational change → G-protein alpha subunit swaps GDP for GTP and detaches

G-protein & GTP go phosphorylate adenylate cyclase

activated Adenylate cyclase converts ATP to cAMP, a secondary messenger which relays signal to cellular machinery

When G-protein hydrolyzes GTP back to GDP it reassociates into inactive trimer and interacts with GPCR to release ligand

What is a receptor tyrosine kinase, and how does it work?

Enzyme-linked receptor that binds growth hormones

Ligand binds → 2 RTKs cross-link and dimerize → tyrosine residues get phosphorylated by the other RTK → phosphorylated tyrosines serve as binding sites for signaling proteins (help shi phosphorylate and activate each other i guess)

Give roles of these organelles:

Nucleus, Mitochondria, Rough ER, Smooth ER, Golgi apparatus, Lysosome, Peroxisome

Nucleus contains DNA

Mitochondria does cellular respiration to produce ATP

Rough ER makes proteins, especially for export

Smooth ER produces lipids and does detox

Golgi apparatus modifies & ships proteins

Lysosome breaks down proteins

Peroxisome breaks down lipids & contains oxidative enzymes

What are the 3 main components of the cytoskeleton? What are their component materials and characteristic functions?

Microfilaments, Intermediate filaments, Microtubules

Microfilaments: made of actin, do cytokinesis, phagocytosis and muscle contraction

Intermediate filaments: material depends on cell (eg. keratin, lamin, desmin). Mostly provide structural integrity, but also help w/ cell migration & subcellular organization

Gram-positive vs. Gram-negative bacteria

Thick peptidoglycan cell wall vs. Thin peptidoglycan cell wall and outer membrane w/ lipoproteins & polysaccharides

Binary fission vs. Mitosis

Binary fission does not involve mitotic spindle, binary fission has DNA duplicate during replication instead of before

4 methods of prokaryotic genetic variation

transformation, conjugation, transduction, transposable elements

Describe and name the 2 ‘processes’ that prevent our body’s stem cell supplies from being depleted.

Obligate asymmetric division = stem cells divide to produce 1 differentiated cell and 1 ‘replacement’ stem cell

Stochastic differentiation = when a stem cell divides to produce 2 differentiated cells, a neighboring stem cell takes notice and divides to produce 2 ‘replacement’ stem cells